A differential gear device used for cars is well known in the prior art as an essential part of the car construction, wherein the differential gear device automatically operates according to the difference between both loads of the left and right driving wheels. However, the differential rate of the device is not adjustable to the varied driving modes and is set on a static condition because of its geared mechanism.
Therefore, in order to maintain a straight driving mode at a high speed, for instance, means for a differential lock may be further included for car construction. Generally, a transmission system for a car includes a complicated oil-control system capable of adjusting the differential rate corresponding to the varied driving modes.
A differential gear device is also used for a car having four driving wheels in order to distribute torque to the front and rear wheels with the aid of an oil-control system adapted to control the torque distributing rate according to the driving mode.
Namely, the car construction including said differential gear device must use an expensive, sophisticated and complicated control system to adjust the differential rate or the torque distributing rate.
A differential rotation control device with a hydraulic assembly is disclosed in U.S. Pat. No. 3,065,700. Such differential rotation control device can be controlled so as to regulate fluid flow between hydrostatic pumps and hydrostatic motors by means for varying inclinations of a pair of swash plates, but it operates with a low efficiency. If this device is applied to a car drive control, when it is driven so as to transmit the most torque power in a small reduction ratio, the pump assemblies must be driven under the highest rotation speed. This causes a significant consumption of energy for pumping fluid at its highest delivery amount and, as a result, lowers the torque transmission efficiency. This is a significant disadvantage of such a prior art device.